Freeze-fracture electron microscopy has revealed organized sites of aggregated intramembranous particles in granular cell luminal membranes from toad urinary bladder after vasopressin (ADH) treatment. This response appears to be a primary hormonal effect which is reversible and quantitatively and specifically related to induced alterations in membrane water permeability. A similar morphologic response, which may have similar physiologic significance, is seen in papillary collecting duct luminal membrane from kidneys of rats. The proposed research is to investigate mechanisms underlying these ADH-associated changes in membrane structure; to establish more fully the physiologic significance of these changes; and, with this morphologic change as an index of membrane water permeability, to consider new concepts related to epithelial transport and renal pathophysiology.